<!DOCTYPE html>
<html>
<head>
    <style>
        div { width:100px; height:80px; margin:5px; float:left; background:#b9e; }
        span { padding: 2px; margin: 3px; line-height: 1.4; border: 1px solid #000; }
    </style>
    <script src="http://code.jquery.com/jquery-latest.js"></script>
</head>
<body>
<div>
    <button>Sibling!</button>
    <button>Sibling!</button>
</div>

<div>
    <button>Sibling!</button>
</div>
<div>
    None
</div>

<div>
    <button>Sibling!</button>
    <span>Sibling!</span>
    <span>Sibling!</span>

</div>
<div>
    <button>Sibling!</button>
</div>
<script>
    $("button:only-of-type").text("Alone").css("border", "2px blue solid");
</script>

</body>
</html>